The Swedish Twin Registry Past and future use

Abstract

The Swedish Twin Registry of the Karolinska Institute comprises about 10 000 sets of twins, and was built up in 1961-1962. It covers about 95% of all Swedish same-sexed twins that were born in the country in 1886-192 5 and were still living as unbroken pairs at the time of compilation.The compilation procedure has been presented in detail by Cederlöf (1966a), and will be summarized as follows. Since the middle of the last century, a rather unique system for population registration has existed in Sweden. It is based not only on birth records, but also on migration records. As soon as an individual moves away from the place he lives, he has to notify the authorities as to where he moves. This system made it possible for us to follow-up any individual from his place of birth to his next place of living, and so on, until we found his current address or had to discard him as deceased. We started, in that way, with about 41 000 multiple birth records, handscreened from about 3½ million birth records from all Sweden during the period in question, and ended with a series of about 12 000 located sets of twins. Of the remainder, about 26 000 had to be discarded, due to death of one or both partners; 1 000, due to emigration to foreign countries; and 100, due to hospitalization in mental asylums, or home for aged people, etc. In the compilation procedure we lost track of about 1 100 individuals, or 1.5%.The zygosity of the twins has been determined with the aid of some similarity questions contained in a questionnaire, mailed to all the twins in the registry. Out of several questions, one proved to be of high reliability, namely the very simple question whether the twins as children were as alike as two peas in a pod, or of family likeness only. We assumed that if both twins in a set agreed in saying that they had been as alike as two peas in a pod, the pair was MZ. If they, on the other hand, agreed in saying that they had been of family likeness only, we regarded the pair as DZ. We validated this questionnaire diagnosis by analyzing the outcome from a subsample of 200 pairs who had also been examined serologically in regard to five markers, A1A2BO, MN, Rh, Hp, and Gm. The results were reported here in Rome in 1961 and showed an agreement in diagnoses of 95% for MZ and 90% for DZ (Cederlöf et al, 1061).